Brain Speed

John K Clark (
Wed, 13 Aug 1997 10:26:13 -0700 (PDT)


On Wed, 13 Aug 1997 Anders Sandberg <> Wrote:

>if you myelinate the axon, the signal can move as a purely
>electrical signal (a change in trans-membrane potential) much faster,
>although it decays exponentially. So most axons are covered with
>myelin except at the nodes of Ravnier, where there are short
>stretches of unmyelinated surface. The result is that signals "jump"
>from one node to the other, and are amplified there so that they can
>reach the next. Still limited, but faster.

It's true that sometimes signals in neurons, particularly in dendrites,
travel passively like in a simple wire, however I don't see how that could
make much of a difference in the overall speed of the brain. Such signals
may not need to wait around to be amplified by voltage gated ion channels,
but they're still pretty slow for other reasons. The neural membrane has a
very high electrical capacitance, and this dramatically slows the signal
because the membrane potential has to build up from the ion flow, and the
capacitance is so big this takes a long time, like filling a swimming pool
with a straw.

Even if such signals were fast it still wouldn't be very important because of
their very short range. After about a millimeter the same high capacitance
has turned a sharp spike into a hill that is so gentle it's unrecognizable.
That's why amplification is needed for most things and that's really slow.

John K Clark

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